HALF-HOURS WITH THE PLANETS.
In observing the stars, we can select a part of the heavens which may be
conveniently observed; and in this way in the course of a year we can
observe every part of the heavens visible in our northern hemisphere.
But with the planets the case is not quite so simple. They come into
view at no fixed season of the year: some of them can never be seen _by
night_ on the meridian; and they all shift their place among the stars,
so that we require some method of determining where to look for them on
any particular night, and of recognising them from neighbouring fixed
stars.
The regular observer will of course make use of the 'Nautical Almanac';
but 'Dietrichsen and Hannay's Almanac' will serve every purpose of the
amateur telescopist. I will briefly describe those parts of the almanac
which are useful to the observer.
It will be found that three pages are assigned to each month, each page
giving different information. If we call these pages I. II. III., then
in order that page I. for each month may fall to the left of the open
double page, and also that I. and II. may be open together, the pages
are arranged in the following order: I. II. III.; III. I. II.; I. II.
III.; and so on.
Now page III. for any month does not concern the amateur observer. It
gives information concerning the moon's motions, which is valuable to
the sailor, and interesting to the student of astronomy, but not
applicable to amateur observation.
[Illustration: PLATE VI.]
We have then only pages I. and II. to consider:--
Across the top of both pages the right ascension and declination of the
planets Venus, Jupiter, Mars, Saturn, Mercury, and Uranus are given,
accompanied by those of two conspicuous stars. This information is very
valuable to the telescopist. In the first place, as we shall presently
see, it shows him what planets are well situated for observation, and
secondly it enables him to map down the path of any planet from day to
day among the fixed stars. This is a very useful exercise, by the way,
and also a very instructive one. The student may either make use of the
regular maps and mark down the planet's path in pencil, taking a light
curve through the points given by the data in his almanac, or he may lay
down a set of meridians suited to the part of the heavens traversed by
the planet, and then proceed to mark in the planet's path and the stars,
taking the latter either from his maps or from a convenient list of
stars.[9] My 'Handbook of the Stars' has been constructed to aid the
student in these processes. It must be noticed that old maps are not
suited for the work, because, through precession, the stars are all out
of place as respects R.A. and Dec. Even the Society's maps, constructed
so as to be right for 1830, are beginning to be out of date. But a
matter of 20 or 30 years either way is not important.[10] My Maps,
Handbook and Zodiac-chart have been constructed for the year 1880, so as
to be serviceable for the next fifty years or so.
Next, below the table of the planets, we have a set of vertical
columns. These are, in order, the days of the month, the calendar--in
which are included some astronomical notices, amongst others the
diameter of Saturn on different dates, the hours at which the sun rises
and sets, the sun's right ascension, declination, diameter, and
longitude; then eight columns which do not concern the observer; after
which come the hours at which the moon rises and sets, the moon's age;
and lastly (so far as the observer is concerned) an important column
about Jupiter's system of satellites.
Next, we have, at the foot of the first page, the hours at which the
planets rise, south, and set; and at the foot of the second page we have
the dates of conjunctions, oppositions, and of other phenomena, the
diameters of Venus, Jupiter, Mars, and Mercury, and finally a few words
respecting the visibility of these four planets.
After the thirty-six pages assigned to the months follow four (pp.
42-46) in which much important astronomical information is contained;
but the points which most concern our observer are (i.) a small table
showing the appearance of Saturn's rings, and (ii.) a table giving the
hours at which Jupiter's satellites are occulted or eclipsed, re-appear,
&c.
We will now take the planets in the order of their distance from the
sun: we shall see that the information given by the almanac is very
important to the observer.
Mercury is so close to the sun as to be rarely seen with the naked eye,
since he never sets much more than two hours and a few minutes after the
sun, or rises by more than that interval before the sun. It must not be
supposed that at each successive epoch of most favourable appearance
Mercury sets so long after the sun or rises so long before him. It would
occupy too much of our space to enter into the circumstances which
affect the length of these intervals. The question, in fact, is not a
very simple one. All the necessary information is given in the almanac.
We merely notice that the planet is most favourably seen as an evening
star in spring, and as a morning star in autumn.[11]
The observer with an equatorial has of course no difficulty in finding
Mercury, since he can at once direct his telescope to the proper point
of the heavens. But the observer with an alt-azimuth might fail for
years together in obtaining a sight of this interesting planet, if he
trusted to unaided naked-eye observations in looking for him. Copernicus
never saw Mercury, though he often looked for him; and Mr. Hind tells me
he has seen the planet but once with the naked eye--though this perhaps
is not a very remarkable circumstance, since the systematic worker in an
observatory seldom has occasion to observe objects with the unaided eye.
By the following method the observer can easily pick up the planet.
Across two uprights (Fig. 10) nail a straight rod, so that when looked
at from some fixed point of view the rod may correspond to the sun's
path near the time of observation. The rod should be at right-angles to
the line of sight to its centre. Fasten another rod at right angles to
the first. From the point at which the rods cross measure off and mark
on both rods spaces each subtending a degree as seen from the point of
view. Thus, if the point of view is 9-1/2 feet off, these spaces must
each be 2 inches long, and they must be proportionately less or greater
as the eye is nearer or farther.
[Illustration: _Fig. 10._]
Now suppose the observer wishes to view Mercury on some day, whereon
Mercury is an evening star. Take, for instance, June 9th, 1868. We find
from 'Dietrichsen' that on this day (at noon) Mercury's R.A. is 6h. 53m.
23s.: and the sun's 5h. 11m. 31s. We need not trouble ourselves about
the odd hours after noon, and thus we have Mercury's R.A. greater than
the sun's by 1h. 41m. 52s. Now we will suppose that the observer has so
fixed his uprights and the two rods, that the sun, seen from the fixed
point of view, appears to pass the point of crossing of the rods at
half-past seven, then Mercury will pass the cross-rod at 11m. 52s. past
nine. But where? To learn this we must take out Mercury's declination,
which is 24° 43' 18" N., and the sun's, which is 22° 59' 10" N. The
difference, 1° 44' 8" N. gives us Mercury's place, which it appears is
rather less than 1-3/4 degree north of the sun. Thus, about 1h. 42m.
after the sun has passed the cross-rod, Mercury will pass it between the
first and second divisions above the point of fastening. The sun will
have set about an hour, and Mercury will be easily found when the
telescope is directed towards the place indicated.
It will be noticed that this method does not require the time to be
exactly known. All we have to do is to note the moment at which the sun
passes the point of fastening of the two rods, and to take our 1h. 42m.
from that moment.
This method, it may be noticed in passing, may be applied to give
naked-eye observations of Mercury at proper seasons (given in the
almanac). By a little ingenuity it may be applied as well to morning as
to evening observations, the sun's passage of the cross-rod being taken
on one morning and Mercury's on the next, so many minutes _before_ the
hour of the first observation. In this way several views of Mercury may
be obtained during the year.
Such methods may appear very insignificant to the systematic observer
with the equatorial, but that they are effective I can assert from my
own experience. Similar methods may be applied to determine from the
position of a known object, that of any neighbouring unknown object even
at night. The cross-rod must be shifted (or else two cross-rods used)
when the unknown _precedes_ the known object. If two cross-rods are
used, account must be taken of the gradual diminution in the length of a
degree of right ascension as we leave the equator.
Even simpler methods carefully applied may serve to give a view of
Mercury. To show this, I may describe how I obtained my first view of
this planet. On June 1st, 1863, I noticed, that at five minutes past
seven the sun, as seen from my study window, appeared from behind the
gable-end of Mr. St. Aubyn's house at Stoke, Devon. I estimated the
effect of Mercury's northerly declination (different of course for a
vertical wall, than for the cross-rod in fig. 8, which, in fact, agrees
with a declination-circle), and found that he would pass out opposite a
particular point of the wall a certain time after the sun. I then turned
the telescope towards that point, and focussed for distinct vision of
distant objects, so that the outline of the house was seen out of focus.
As the calculated time of apparition approached, I moved the telescope
up and down so that the field swept the neighbourhood of the estimated
point of apparition. I need hardly say that Mercury did not appear
exactly at the assigned point, nor did I see him make his first
appearance; but I picked him up so soon after emergence that the outline
of the house was in the field of view with him. He appeared as a
half-disc. I followed him with the telescope until the sun had set, and
soon after I was able to see him very distinctly with the naked eye. He
shone with a peculiar brilliance on the still bright sky; but although
perfectly distinct to the view when his place was indicated, he escaped
detection by the undirected eye.[12]
Mercury does not present any features of great interest in ordinary
telescopes; though he usually appears better defined than Venus, at
least as the latter is seen on a dark sky. The phases are pleasingly
seen (as shown in Plate 6) with a telescope of moderate power. For their
proper observation, however, the planet must be looked for with the
telescope in the manner above indicated, as he always shows a nearly
semi-circular disc when he is visible to the naked eye.
We come next to Venus, the most splendid of all the planets to the eye.
In the telescope Venus disappoints the observer, however. Her intense
lustre brings out every defect of the instrument, and especially the
chromatic aberration. A dark glass often improves the view, but not
always. Besides, an interposed glass has an unpleasant effect on the
field of view.
Perhaps the best method of observing Venus is to search for her when she
is still high above the horizon, and when therefore the background of
the sky is bright enough to take off the planet's glare. The method I
have described for the observation of Mercury will prove very useful in
the search for Venus when the sun is above the horizon or but just set.
Of course, when an object is to be looked for high above the horizon,
the two rods which support the cross-rods must not be upright, but
square to the line of view to that part of the sky.
But the observer must not expect to see much during his observation of
Venus. In fact, he can scarcely do more than note her varying phases
(see Plate 6) and the somewhat uneven boundary of the terminator. Our
leading observers have done so little with this fascinating but
disappointing planet, that amateurs must not be surprised at their own
failure.
I suppose the question whether Venus has a satellite, or at any rate
whether the object supposed to have been seen by Cassini and other old
observers were a satellite, must be considered as decided in the
negative. That Cassini should have seen an object which Dawes and Webb
have failed to see must be considered utterly improbable.
Leaving the inferior planets, we come to a series of important and
interesting objects.
First we have the planet Mars, nearly the last in the scale of planetary
magnitude, but far from being the least interesting of the planets. It
is in fact quite certain that we obtain a better view of Mars than of
any object in the heavens, save the Moon alone. He may present a less
distinguished appearance than Jupiter or Saturn, but we see his surface
on a larger scale than that of either of those giant orbs, even if we
assume that we ever obtain a fair view of their real surface.
Nor need the moderately armed observer despair of obtaining interesting
views of Mars. The telescope with which Beer and Mädler made their
celebrated series of views was only a 4-inch one, so that with a 3-inch
or even a 2-inch aperture the attentive observer may expect interesting
views. In fact, more depends on the observer than on the instrument. A
patient and attentive scrutiny will reveal features which at the first
view wholly escape notice.
In Plate 6 I have given a series of views of Mars much more distinct
than an observer may expect to obtain with moderate powers. I add a
chart of Mars, a miniature of one I have prepared from a charming
series of tracings supplied me by Mr. Dawes. The views taken by this
celebrated observer in 1852, 1856, 1860, 1862, and 1864, are far better
than any others I have seen. The views by Beer and Mädler are good, as
are some of Secchi's (though they appear badly drawn), Nasmyth's and
Phillips'; Delarue's two views are also admirable; and Lockyer has given
a better set of views than any of the others. But there is an amount of
detail in Mr. Dawes' views which renders them superior to any yet taken.
I must confess I failed at a first view to see the full value of Mr.
Dawes' tracings. Faint marks appeared, which I supposed to be merely
intended to represent shadings scarcely seen. A more careful study
shewed me that every mark is to be taken as the representative of what
Mr. Dawes actually saw. The consistency of the views is perfectly
wonderful, when compared with the vagueness and inconsistency observable
in nearly all other views. And this consistency is not shown by mere
resemblance, which might have been an effect rather of memory
(unconsciously exerted) than observation. The same feature changes so
much in figure, as it appears on different parts of the disc, that it
was sometimes only on a careful projection of different views that I
could determine what certain features near the limb represented. But
when this had been done, and the distortion through the effect of
foreshortening corrected, the feature was found to be as true in shape
as if it had been seen in the centre of the planet's disc.
In examining Mr. Dawes' drawings it was necessary that the position of
Mars' axis should be known. The data for determining this were taken
from Dr. Oudemann's determinations given in a valuable paper on Mars
issued from Mr. Bishop's observatory. But instead of calculating Mars'
presentation by the formulæ there given, I found it convenient rather to
make use of geometrical constructions applied to my 'Charts of the
Terrestrial Planets.' Taking Mädler's start-point for Martial
longitudes, that is the longitude-line passing near Dawes' forked bay, I
found that my results agreed pretty fairly with those in Prof. Phillips'
map, so far as the latter went; but there are many details in my charts
not found in Prof. Phillips' nor in Mädler's earlier charts.
I have applied to the different features the names of those observers
who have studied the physical peculiarities presented by Mars. Mr.
Dawes' name naturally occurs more frequently than others. Indeed, if I
had followed the rule of giving to each feature the name of its
discoverer, Mr. Dawes' name would have occurred much more frequently
than it actually does.
On account of the eccentricity of his orbit, Mars is seen much better in
some oppositions than in others. When best seen the southern hemisphere
is brought more into view than the northern because the summer of his
northern hemisphere occurs when he is nearly in aphelion (as is the case
with the Earth by the way).
The relative dimensions and presentation of Mars, as seen in opposition
in perihelion, and in opposition in aphelion, are shown in the two rows
of figures.
In and near quadrature Mars is perceptibly gibbous. He is seen thus
about two months before or after opposition. In the former case, he
rises late and comes to the meridian six hours or so after midnight. In
the latter case, he is well seen in the evening, coming to the meridian
at six. His appearance and relative dimensions as he passes from
opposition to quadrature are shown in the last three figures of the
upper row.
Mars' polar caps may be seen with very moderate powers.
I add four sets of meridians (Plate 6), by filling in which from the
charts the observer may obtain any number of views of the planet as it
appears at different times.
Passing over the asteroids, which are not very interesting objects to
the amateur telescopist, we come to Jupiter, the giant of the solar
system, surpassing our Earth more than 1400 times in volume, and
overweighing all the planets taken together twice over.
Jupiter is one of the easiest of all objects of telescopic observation.
No one can mistake this orb when it shines on a dark sky, and only Venus
can be mistaken for it when seen as a morning or evening star. Sometimes
both are seen together on the twilight sky, and then Venus is generally
the brighter. Seen, however, at her brightest and at her greatest
elongation from the sun, her splendour scarcely exceeds that with which
Jupiter shines when high above the southern horizon at midnight.
Jupiter's satellites may be seen with very low powers; indeed the outer
ones have been seen with the naked eye, and all are visible in a good
opera-glass. Their dimensions relatively to the disc are shown in Plate
7. Their greatest elongations are compared with the disc in the
low-power view.
Jupiter's belts may also be well seen with moderate telescopic power.
The outer parts of his disc are perceptibly less bright than the centre.
More difficult of observation are the transits of the satellites and of
their shadows. Still the attentive observer can see the shadows with an
aperture of two inches, and the satellites themselves with an aperture
of three inches.
The minute at which the satellites enter on the disc, or pass off, is
given in 'Dietrichsen's Almanac.' The 'Nautical Almanac' also gives the
corresponding data for the shadows.
The eclipses of the satellites in Jupiter's shadow, and their
occultations by his disc, are also given in 'Dietrichsen's Almanac.'
In the inverting telescope the satellites move from right to left in the
nearer parts of their orbit, and therefore transit Jupiter's disc in
that direction, and from left to right in the farther parts. Also note
that _before_ opposition, (i.) the shadows travel in front of the
satellites in transiting the disc; (ii.) the satellites are eclipsed in
Jupiter's _shadow_; (iii.) they reappear from behind his _disc_. On the
other hand, _after_ opposition, (i.) the shadows travel _behind_ the
satellites in transiting the disc; (ii.) the satellites are occulted by
the _disc_; (iii.) they reappear from eclipse in Jupiter's _shadow_.
Conjunctions of the satellites are common phenomena, and may be waited
for by the observer who sees the chance. An eclipse of one satellite by
the shadow of another is not a common phenomenon; in fact, I have never
heard of such an eclipse being seen. That a satellite should be quite
extinguished by another's shadow is a phenomenon not absolutely
impossible, but which cannot happen save at long intervals.
The shadows are not _black spots_ as is erroneously stated in nearly all
popular works on astronomy. The shadow of the fourth, for instance, is
nearly all penumbra, the really black part being quite minute by
comparison. The shadow of the third has a considerable penumbra, and
even that of the first is not wholly black. These penumbras may not be
perceptible, but they affect the appearance of the shadows. For
instance, the shadow of the fourth is perceptibly larger but less black
than that of the third, though the third is the larger satellite.
In transit the first satellite moves fastest, the fourth slowest, the
others in their order. The shadow moves just as fast (appreciably) as
the satellite it belongs to. Sometimes the shadow of the satellite may
be seen to overtake (apparently) the disc of another. In such a case the
shadow does not pass over the disc, but the disc conceals the shadow.
This is explained by the fact that the shadow, if visible throughout its
length, would be a line reaching slantwise from the satellite it belongs
to, and the end of the shadow (that is, the point where it meets the
disc) is _not_ the point where the shadow crosses the orbit of any inner
satellite. Thus the latter may be interposed between the end of the
shadow--the only part of the shadow really visible--and the eye; but the
end of the shadow _cannot_ be interposed between the satellite and the
eye. If a satellite _on the disc_ were eclipsed by another satellite,
the black spot thus formed would be in another place from the black spot
on the planet's body. I mention all this because, simple as the question
may seem, I have known careful observers to make mistakes on this
subject. A shadow is seen crossing the disc and overtaking, apparently,
a satellite in transit. It seems therefore, on a first view, that the
shadow will hide the satellite, and observers have even said that they
have _seen_ this happen. But they are deceived. It is obvious that _if
one satellite eclipse another, the shadows of both must occupy the same
point on Jupiter's body_. Thus it is the overtaking of one _shadow_ by
another on the disc, and not the overtaking of a _satellite_ by a
shadow, which determines the occurrence of that as yet unrecorded
phenomenon, the eclipse of one satellite by another.[13]
The satellites when far from Jupiter seem to lie in a straight line
through his centre. But as a matter of fact they do not in general lie
in an exact straight line. If their orbits could be seen as lines of
light, they would appear, in general, as very long ellipses. The orbit
of the fourth would frequently be seen to be _quite clear_ of Jupiter's
disc, and the orbit of the third might in some very exceptional
instances pass _just_ clear of the disc. The satellites move most nearly
in a straight line (apparently) when Jupiter comes to opposition in the
beginning of February or August, and they appear to depart most from
rectilinear motion when opposition occurs in the beginning of May and
November. At these epochs the fourth satellite may be seen to pass above
and below Jupiter's disc at a distance equal to about one-sixth of the
disc's radius.
The shadows do not travel in the same apparent paths as the satellites
themselves across the disc, but (in an inverting telescope) _below_ from
August to January, and _above_ from February to July.
We come now to the most charming telescopic object in the heavens--the
planet Saturn. Inferior only to Jupiter in mass and volume, this planet
surpasses him in the magnificence of his system. Seen in a telescope of
adequate power, Saturn is an object of surpassing loveliness. He must be
an unimaginative man who can see Saturn for the first time in such a
telescope, without a feeling of awe and amazement. If there is any
object in the heavens--I except not even the Sun--calculated to impress
one with a sense of the wisdom and omnipotence of the Creator it is
this. "His fashioning hand" is indeed visible throughout space, but in
Saturn's system it is most impressively manifest.
Saturn, to be satisfactorily seen, requires a much more powerful
telescope than Jupiter. A good 2-inch telescope will do much, however,
in exhibiting his rings and belts. I have never seen him satisfactorily
myself with such an aperture, but Mr. Grover has not only seen the
above-named features, but even a penumbra to the shadow on the rings
with a 2-inch telescope.
Saturn revolving round the sun in a long period--nearly thirty
years--presents slowly varying changes of appearance (see Plate 7). At
one time the edge of his ring is turned nearly towards the earth; seven
or eight years later his rings are as much open as they can ever be;
then they gradually close up during a corresponding interval; open out
again, exhibiting a different face; and finally close up as first seen.
The last epoch of greatest opening occurred in 1856, the next occurs in
1870: the last epoch of disappearance occurred in 1862-63, the next
occurs in 1879. The successive views obtained are as in Plate 7 in order
from right to left, then back to the right-hand figure (but sloped the
other way); inverting the page we have this figure thus sloped, and the
following changes are now indicated by the other figures in order back
to the first (but sloped the other way and still inverted), thus
returning to the right-hand figure as seen without inversion.
The division in the ring can be seen in a good 2-inch aperture in
favourable weather. The dark ring requires a good 4-inch and good
weather.
Saturn's satellites do not, like Jupiter's, form a system of nearly
equal bodies. Titan, the sixth, is probably larger than any of
Jupiter's satellites. The eighth also (Japetus) is a large body,
probably at least equal to Jupiter's third satellite. But Rhea, Dione,
and Tethys are much less conspicuous, and the other three cannot be seen
without more powerful telescopes than those we are here dealing with.
So far as my own experience goes, I consider that the five larger
satellites may be seen distinctly in good weather with a good 3-1/2-inch
aperture. I have never seen them with such an aperture, but I judge from
the distinctness with which these satellites may be seen with a 4-inch
aperture. Titan is generally to be looked for at a considerable distance
from Saturn--_always_ when the ring is widely open. Japetus is to be
looked for yet farther from the disc. In fact, when Saturn comes to
opposition in perihelion (in winter only this can happen) Japetus may be
as far from Saturn as one-third of the apparent diameter of the moon. I
believe that under these circumstances, or even under less favourable
circumstances, Japetus could be seen with a good opera-glass. So also
might Titan.
Transits, eclipses, and occulations of Saturn's satellites can only be
seen when the ring is turned nearly edgewise towards the earth. For the
orbits of the seven inner satellites lying nearly in the plane of the
rings would (if visible throughout their extent) then only appear as
straight lines, or as long ellipses cutting the planet's disc.
The belts on Saturn are not very conspicuous. A good 3-1/2-inch is
required (so far as my experience extends) to show them satisfactorily.
The rings when turned edgewise either towards the earth or sun, are not
visible in ordinary telescopes, neither can they be seen when the earth
and sun are on opposite sides of the rings. In powerful telescopes the
rings seem never entirely to disappear.
The shadow of the planet on the rings may be well seen with a good
2-inch telescope, which will also show Ball's division in the rings. The
shadow of the rings on the planet is a somewhat more difficult feature.
The shadow of the planet on the rings is best seen when the rings are
well open and the planet is in or near quadrature. It is to be looked
for to the left of the ball (in an inverting telescope) at quadrature
preceding opposition, and to the right at quadrature following
opposition. Saturn is more likely to be studied at the latter than at
the former quadrature, as in quadrature preceding opposition he is a
morning star. The shadow of the rings on the planet is best seen when
the rings are but moderately open, and Saturn is in or near quadrature.
When the shadow lies outside the rings it is best seen, as the dark ring
takes off from the sharpness of the contrast when the shadow lies within
the ring. It would take more space than I can spare here to show how it
is to be determined (independently) whether the shadow lies within or
without the ring. But the 'Nautical Almanac' gives the means of
determining this point. When, in the table for assigning the appearance
of the rings, _l_ is less than _l'_ the shadow lies outside the ring,
when _l_ is greater than _l'_ the shadow lies within the ring.
Uranus is just visible to the naked eye when he is in opposition, and
his place accurately known. But he presents no phenomena of interest. I
have seen him under powers which made his disc nearly equal to that of
the moon, yet could see nothing but a faint bluish disc.
Neptune also is easily found if his place be accurately noted on a map,
and a good finder used. We have only to turn the telescope to a few
stars seen in the finder nearly in the place marked in our map, and
presently we shall recognise the one we want by the peculiarity of its
light. What is the lowest power which will exhibit Neptune as a disc I
do not know, but I am certain no observer can mistake him for a fixed
star with a 2-inch aperture and a few minutes' patient scrutiny in
favourable weather.
[Illustration: PLATE VII.]